Previously, in manufacturing a semiconductor device by mounting a semiconductor chip on a wiring board, a wire bonding technique has been employed in which bonding pads on a semiconductor chip and leads on a wiring board are electrically connected by bonding via thin metal wires. Recently, in order to cope with a requirement for downsizing and lightening electrical equipments and an increase in the number of connection terminals of a semiconductor device, a flip chip mounting technology is employed in which a projecting electrode (hereinafter referred to as a “bump”), is formed on an electrode on a surface of a semiconductor chip and, then, the semiconductor chip is directly bonded to a wiring board in a face down manner.
In this flip chip mounting technology, bumps are formed on a plurality of electrodes formed on a semiconductor chip by using a metallic material such as solder and Au, these bumps and corresponding electrodes formed on a wiring board are positioned, and they are bonded by heat-press. In order to enhance a reliability of a semiconductor, an underfill material, which functions as a sealing resin protecting electrodes from adverse environmental effects such as oxidation via oxygen or moisture and which functions as a thermal stress buffer material preventing destruction of bumps with a thermal stress caused by a difference in thermal expansion coefficients between the semiconductor chip and the wiring board during cooling after heat-press, is supplied between the semiconductor chip and the wiring board.
For supplying an underfill material, there are two methods: a method comprising heat-pressing a semiconductor chip on which bumps are formed on a wiring board and, then, filling a liquid resin between the semiconductor chip and the wiring board; and a method comprising supplying first a liquid resin or a resin film onto a wiring board and, then, bonding a semiconductor chip to the wiring board.
However, since any of the above methods requires a step for supplying an underfill material and there are restrictions such as storage and a use limit for the underfill material, problems occur such as a reduction in a working efficiency, a cost increase and the like.
In addition, from the view point how the environmental load is reduced, there is a problem that a product made by bonding many parts such as a mounting board in an electronics field is dismantled into the parts and these parts are recovered to recycle. However, when an underfill material is used, it is difficult to remove it during dismantlement.
Furthermore, due to thinning of a semiconductor device, as a gap between a semiconductor chip and a wiring board is getting narrower and a pitch between wirings formed on the board is getting narrower, it becomes difficult to fill a resin sufficiently between wirings.
Then, it is needed to consider a means for securing reliability of a semiconductor device by preventing electrodes on a semiconductor chip and a wiring board from deteriorating; and buffering a thermal stress between the semiconductor chip and the wiring board without using a resin such as an underfill material.
As a means for buffering a thermal stress between a semiconductor chip having bumps and a wiring board without using an underfill material, for example, Japanese Patent No. 3116926 discloses in its Specification a structure in which a low-elastic layer is provided beneath an area around bumps formed on a semiconductor chip to buffer a thermal stress.
In addition, a thermal stress is buffered by investing bumps themselves with elasticity. For example, JP-A No. 1999-214447 and JP-A No. 2001-156091 disclose a structure in which a thermal stress is buffered by forming a cavity within a solder bump. JP-A No. 1999-233669 discloses a structure in which a thermal stress is buffered by utilizing elasticity of a resin by forming a bump consisting of a resin core made of a photo-sensitive resin such as polyimide, acrylic and the like and Ni plating and the like on a surface of the core. JP-A No. 2000-320148 discloses a structure in which a thermal stress generated between an integrated circuit and a mounting board is buffered by utilizing a U-shape elastic element at solder joints.
Further, for bonding a fine pattern below a 10 μm pitch, since bump connecting becomes difficult and when heterogeneous materials exist in a joint, properties of the materials in the joint change due to a diffusion reaction during bonding so that reliability cannot be secured, a need arises to adopt a bumpless connection in which electrodes formed on a semiconductor chip and electrodes formed on a wiring board are directly contacted while excluding heterogeneous materials in the joint.
In the case where substrates made of the same material are bonded (e.g., Si chips are bonded or a Si chip is mounted on a Si interposer substrate), since a stress applied to a joint in a bumpless connection should be buffered by bending of both two substrates, these substrates are thinned as far as possible until they obtain elasticity.
Currently, Si thin wafers with a thickness of 50 μm are produced in a mass-production, and a wafer with a thickness of 30 μm or thinner has been developed.
In the above conventional technology, although a stress-buffering function as which an underfill material has can be achieved without using such an underfill material, a sealing function to prevent deterioration of electrodes has not been discussed.
Further, in a field of electronics, as problems from a short term viewpoint with respect to yielding improvement such as repairing and reworking, and problems from a long term viewpoint with respect to the basis of industry in a circular economic society such as recycling and reusing, a development of a technology for removably mounting has become considered important. However, since in a conventional mounting technology a semiconductor device is made by bonding electrodes on a semiconductor chip and electrodes on a wiring board, electrodes are destroyed when the semiconductor chip is removed from the wiring board and, thus, it is difficult to reuse the semiconductor chip and the wiring board.